1. Field of the Invention
The present invention relates to a biodegradable plastics composition, a molded article of the composition, and a method of controlling a degradation rate, and more specifically, to a biodegradable plastics composition in which a biodegradable plastics is blended with three kinds of specific additives so that its biodegradation rate is controlled, its hydrolysis rate is retarded, and its weatherability is significantly improved by the synergistic effect of the additives, a molded article of the composition, and a method of controlling the degradation rate of the biodegradable plastics.
2. Description of the Related Art
In recent years where a problem such as environmental pollution due to plastics wastes is zoomed in on, the frequency at which a biodegradable plastics is used has been increasing, and the modification of the plastics has been advanced owing to increasing need for, and increasing significance of, environmental protection.
The biodegradable plastics are roughly classified into aliphatic polyesters having ester bonds in a molecular backbone thereof (including those produced by microorganisms), natural macromolecule type polymers such as cellulose, polysaccharides produced by microorganisms, polyvinyl alcohol (PVA), and polyethers such as polyethylene glycol (PEG).
Of those, aliphatic polyesters have not been utilized as the biodegradable plastics because they have, in general, low melting points and poor heat stability in their production stages, and a molecular weight to be obtained is not large enough to provide physical properties suitable for practical molded articles. With technological developments to solve these problems, a high molecular weight of aliphatic polyester has come out, and started to be used in such fields as materials for agriculture, forestry, and fishery (agricultural mulch film, plantation pot, fishing line, fishing net, and the like), materials for civil engineering work (water holding sheet, net for plants, sandbag, and the like), and packaging and containers (those difficult to be recycled due to adhered soil, foods, and the like).
A biodegradable plastics typified by the above polymeric aliphatic polyester is preferably degraded quickly by microorganisms widely present in the natural world after having been disposed of. However, when a biodegradable plastics is put into widespread use so as to be utilized as an alternative to a conventional plastics, in particular, as a consumer durable, it is indispensable to cause the biodegradable plastics to retain functions such as weatherability, water resistance, heat resistance, and mechanical properties as high as those of the conventional plastics during the long-term use of the biodegradable plastics. It has been sufficient for a conventional biodegradable plastics used in a consumer durable to retain such functions for about five years. In recent years, however, a biodegradable plastics capable of retaining such functions for about ten years or longer has been requested.
A method involving adjusting the biodegradation rate of a biodegradable plastics to improve the resistance of the biodegradable plastics to hydrolysis is one method of causing the biodegradable plastics to retain functions as high as those of a conventional plastics during the long-term use of the biodegradable plastics. As an example of the method, JP 11-80522 A, for example, proposes a biodegradable plastics composition in which a biodegradable plastics is blended with a carbodiimide compound so that its biodegradation rate is controlled. However, the composition has an insufficient function of adjusting the hydrolysis rate or biodegradation rate of the plastics in some cases.
Further, JP 2001-525473 A, for example, proposes a biodegradable plastics added with an aromatic carbodiimide as a hydrolysis stabilizer so that the degradation of the plastics is retarded. However, the plastics involves a problem in that resultant products therefrom lose its transparency irrespective of the amount in which the stabilizer is added.
Moreover, JP 06-184417 A, for example, proposes a lactic acid-based composition and the like in which a lactic acid-based polymer as a biodegradable plastics is blended with at least one type of additive selected from the group consisting of ultraviolet absorbents and light stabilizers. However, this invention is aimed at controlling decomposition of the polymer by solar rays or the like, but not at controlling biodegradation.
Meanwhile, JP 2002-114893 A, for example, proposes a thermoplastic polyester resin composition, which is not biodegradable, in which a thermoplastic polyester resin is compounded with a resin-based ultraviolet absorbent and aliphatic-based polycarbodiimide compound to improve resistances of the resin, which has a relatively high heat resistance, weatherability, alkali or hydrolysis resistance. However, the resin-based ultraviolet absorbent disclosed in JP 2002-114893 A involves a problem of being not compatible with a thermoplastic polyester resin.
To solve the above problems, JP 2003-313436 A, for example, proposes a biodegradable plastics composition obtained by blending a biodegradable plastics with a carbodiimide compound and a hindered phenol-based antioxidant.
In addition, JP 2004-155993 A, for example, proposes a biodegradable plastics composition obtained by blending a biodegradable plastics with a carbodiimide compound and at least one kind of a compound selected from a benzotriazole-based compound, a triazine-based compound, and a hydroxylamine-based compound.
Further, JP2005-82642A, for example, proposes a biodegradable plastics composition using a hydrolysis-resistant stabilizer composed of a carbodiimide composition obtained by blending an aliphatic carbodiimide compound with a phosphorus-based antioxidant.
However, the resistance of each of the above compositions to hydrolysis is insufficient in some cases.
Moreover, none of the respective patent documents described above describes that both an ultraviolet absorbent (hereinafter, referred to a UV absorbent) and an antioxidant are actively used in combination with a carbodiimide compound, and an effect by the combined use.
In addition, in particular, biodegradable plastics derived from natural products among the biodegradable plastics are produced from reproducible organic resources, and have been recently attracting attention from the viewpoints of the prevention of global warming and the establishment of a cyclic society. However, there are no descriptions concerning a fact that both an ultraviolet absorbent and an antioxidant, and a carbodiimide compound should be actively blended together into any one of those biodegradable plastics derived from natural products, and an effect exerted by the blending.